Bone screws are implantable medical fixation devices used in many fields of surgery, like traumatology, spine surgery and dental surgery. For different reasons, the insertion torque is an important factor for success of the treatment. In spinal surgery for example, a maximal insertion torque is defined to prevent bone from cracking. In traumatology another reason for screw insertion with a controlled torque is the prevention of cold-welding between plates and screws, which renders implant removal impossible.
In dental applications, a too high torque indicates a not correctly prepared bone bed, which might influence osseo-integration of an implant. Furthermore, often a limited torque is required such as to prevent damage to the drive in a screw or similar, such as to allow a subsequent removal of the screw or similar.
The insertion torque usually needs to be controlled to prevent a reduced performance of a medical device, either relating to its direct interaction with bone or its interaction with another medical device of a larger implant system or assembly.
To control these insertion torques, torque wrenches are available on the market. Common torque wrenches often are very complexly built, expensive and need regular calibration to guarantee the desired torque. The complexity of the existing devices often influences the cleanability of these surgical instruments. Furthermore, torque wrenches are expensive devices and most often only are designed to control one level of torque per wrench type.
As a further example, EP 1 854 587 A2 (Teleflex Medical Inc.) describes a magnetic torque-limiting device. The device comprises a first end to engage a fastener, a second end to receive an applied torque and a magnetic torque limiter to transmit the applied torque from the second end to the first end such as to rotate the first end when the applied torque is less than a predetermined limit. The device includes a pair of magnets aligned with opposing poles facing one another. When the handle is rotated, one or more lugs are brought to bear on levers. The levers include an arm to urge the magnets apart as well as an arm bearing on a lug. When the force exerted on the lugs exceeds the magnetic force, the magnets will be urged to separate by means of the levers and the torque transmitted to the first end is diminished.
WO 2013/088158 (Kennedy et al.) discloses an adjustment device comprising a handle portion, a spindle portion adapted to engage an element to be rotated against resistive torque, a first magnet arrangement comprising one or more magnets and being attached to the handle portion as well as a second magnet arrangement comprising one or more permanent magnets and being attached to the spindle portion. The distance between the first and the second magnet arrangement may be increased or decreased, whereby the magnets are arranged such that poles of one magnet arrangement are generally aligned with opposing poles of the second magnet arrangement. Hence, rotation of one of the magnet arrangements entails the rotation of the second magnet arrangement. If a pre-set torque level is reached, the magnetic attraction forces are overcome by the rotational forces, such that the entailed rotation of the magnet arrangement is stopped.
U.S. Pat. No. 5,655,420 (Ogawa et al.) describes tightening tools having a defined maximal torque such as to avoid the problem of over tightening pipe joints. The tightening tool comprises a head having a socket for an engagement portion of a threaded member, a head supporting the head rotatably about the axis of the socket and a handle integral with the head support, a magnet being attached to one of the head and the head support, while an attracted portion is provided on the other and attractable to the magnet. The position of the magnet is made adjustable such as to vary the distance between the magnet and the attracted portion.
One major drawback of the torque limiter arrangements known in the prior art is that they require different separate parts cooperating with each other, some of these parts even being movable in respect to other parts. Especially in connection with surgical instruments, such as surgical screw drivers, assemblies of different parts and movable parts may not be easily cleaned and sterilized, especially since body fluids or tissue parts may remain stuck between assembled parts or jammed under a movable part. However, insufficient cleaning and sterilization may be a serious threat to patients subsequently treated with said instruments. Further, either the torque limiter according to the prior art only have a single pre-defined torque level or the switching to a different torque level requires time-consuming manual adjustment.